DIFP (Diisopropylfluorophosphate(
 
Diisopropylfluorophosphate, also known as diisopropylphosphofluoridate, DIPF, and sometimes abbreviated DFP, was developed as a nerve gas during the World War II. It is an irreversible inhibitor of acetylcholinesterase and, more generally, an inhibitor of serine proteinases (1). In chymotrypsin, a typical serine proteinase, the active site Ser195 is phosphorylated by DIFP in a reaction that mimics the catalytic mechanism (Fig. 1). None of the other 27 serine residues of chymotrypsin is affected. The resultant diisopropylphosphoryl serine is stable to subsequent hydrolysis, and the phosphorylated enzyme is totally inactive. The reagent is a liquid at room temperature and is usually diluted into isopropanol before use (2). It is volatile and highly toxic: LD₅₀ in mice is 3.7 mg/kg when administered orally. A more convenient reagent for inactivating serine proteinases is phenylmethane sulfonyl fluoride (PMSF) (3). 

Figure 1. Reaction of the hydroxyl group of the active site serine residue of a serine proteinase with diisopropyl fluorophosphate. The product is catalytically inactive.
 
References
1. J. A. Cohen, R. A. Oosterbaan, and F. Berends (1967) Methods Enzymol. 11, 686–702. 
2. R. J. Beynon and J. S. Bond (1993) In Proteolytic Enzymes: A Practical Approach (R. J. Beynon and J. S. Bond, eds.), IRL Press, Oxford, U.K., p. 243. 
3. B. M. Dunn (1993) In Proteolytic Enzymes: A Practical Approach (R. J. Beynon and J. S. Bond, eds.), IRL Press, Oxford, U.K., pp. 57–81.